[RFC] readtags: supports multiple tag files

[masatake]

This pull request implements the ideas I wrote in https://github.com/universal-ctags/citre/issues/178.

Example sessions:

• https://github.com/universal-ctags/citre/issues/178#issuecomment-2330336232
• https://github.com/universal-ctags/citre/issues/178#issuecomment-2330430958

With this pull request, readtags can take multiple -t option. So you can find a tag in multiple files at once. Using multiple threads is future work.

This pull request adds !_READTAGS_INCLUDE directive. If a tag file is specified with the pseudo tag, readtags reads the tas file, too, as if the tag file is specified with -t on the command line.

This pull request adds -X/--generate-aggregate-tag-file option to readtags. With the option, readtags generate an aggregate tag file enumerating all tag files specified with -t on the command line.

$ ./readtags -t podman.tags -t glibc.tags -t coreutils832.tags -X           
!_READTAGS_INCLUDE	podman.tags	//
!_READTAGS_INCLUDE	glibc.tags	//
!_READTAGS_INCLUDE	coreutils832.tags	//
!_TAG_FILE_FORMAT	2	//
!_TAG_FILE_SORTED	1	//
!_TAG_OUTPUT_EXCMD	pattern	//
!_TAG_OUTPUT_FILESEP	slash	//
!_TAG_OUTPUT_MODE	u-ctags	//
!_TAG_OUTPUT_VERSION	0.0	/current.age/
!_TAG_PATTERN_LENGTH_LIMIT	0	//
!_TAG_PROC_CWD	/home/yamato/var/ctags-github	//
!_TAG_PROGRAM_AUTHOR	Universal Ctags Team	//
!_TAG_PROGRAM_NAME	readtags	/with -X option/
!_TAG_PROGRAM_URL	https://ctags.io/	/official site/
!_TAG_PROGRAM_VERSION	0.0.0	/TODO/

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.34783% with 18 lines in your changes missing coverage. Please review.

Project coverage is 85.53%. Comparing base (5039ab9) to head (953f035). Report is 441 commits behind head on master.

Files with missing lines	Patch %	Lines
extra-cmds/readtags-cmd.c	84.34%	18 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4068      +/-   ##
==========================================
+ Coverage   85.51%   85.53%   +0.02%     
==========================================
  Files         237      237              
  Lines       57030    57030              
==========================================
+ Hits        48769    48782      +13     
+ Misses       8261     8248      -13     

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